Imagine a situation in which the same body is simultaneously in motion and at rest.

According to Newton’s first law, any body that moves rectilinearly and uniformly can be considered as being at rest, and vice versa – everything depends only on the chosen reporting system.

For example, a train moves straight and uniformly with a speed v = 30 km / h. If we consider its movement relative to the communication post near the road, then we can say that it is moving. However, if we consider its movement relative to the book that lies on the shelf of the train, then we can say that the train is at rest.